The present invention relates to a new process for isolating polyisocyanate mixtures in pure or concentrated form from polyisocyanate mixtures containing at least two polyisocyanate isomers.
It is known that polyisocyanate mixtures can be purified by fractional distillation. For example, in DE-OS 3,145,010, the isolation of high-purity 4,4'-diisocyanato-diphenylmethane (hereinafter referred to as "4,4'-MDI") from a polyisocyanate mixture such as is obtained by phosgenation of aniline/formaldehyde condensation products is disclosed. Disadvantages of this five-stage distillation process are the expense of the apparatus for the multiple distillation of the polyisocyanate mixture and, associated therewith, the high thermal stress of the process product, 4,4'-MDI.
The separation of 2,4'-diisocyanato-diphenylmethane (hereinafter "2,4'-MDI") and 4,4'-MDI from polyisocyanate mixtures of the type described in DE-OS 3,145,010 by distillation and subsequent crystallization is disclosed in DE-OS 2,425,658. In this disclosed process, a distillate containing 2,4'-MDI and 4,4'-MDI is first obtained from the crude polyisocyanate mixture. Cooling of this distillate to a temperature of from 40.degree. C. to 18.degree. C. for 5-6 hours results in the crystallization of high purity 4,4'-MDI. This process is disadvantageous because of the long cooling-down time required for the crystallization and, associated therewith, the strict temperature control requirements.
The continuous separation of polyisocyanate isomer mixtures of tolylene 2,4-diisocyanate (hereinafter "2,4-TDI") and tolylene 2,6-diisocyanate (hereinafter "2,6-TDI") by partial crystallization in a tubular crystallizer is described in DE-OS 3,220,439. In this disclosed process, the isomer mixture is cooled from 30.degree. C. to 6.degree. C. in 5 hours by passing it through a cooled tube. A 2,4-TDI-rich product is obtained after the crystallizate is melted at 20.degree. C. and the mother liquor is drained off. The mother liquor can be further purified. A disadvantage of this process is the long time necessary for the crystallization.
U.S. Pat. No. 4,246,187 discloses a process which is based on the same principle as that disclosed in DE-OS 3,220,439. In the process disclosed in U.S. Pat. No. 4,246,187, the isomer mixture of 2,4-TDI and 2,6-TDI is cooled to a temperature near 6.degree. C. in a heat exchanger in the form of a skimmer centrifuge. The mother liquor obtained is centrifuged off and the crystallizate is continuously discharged. A disadvantage of this process is the technically demanding continuous discharge of solids from the reaction mixture.
In the process described in published Application JP 49 003 981, the stereoisomers of 4,4'-diisocyanato-dicyclohexylmethane (hereinafter "H.sub.12 -MDI"), that is cis,cis-, cis,trans- and trans,trans-isomers, are separated from aliphatic hydrocarbons by fractional crystallization. To this end, the stereoisomer mixture is dissolved completely at temperatures near 40.degree. C. in the solvent used, and the trans,trans-rich H.sub.12 -MDI isomer mixture crystallizing out on cooling to 10.degree. C. is isolated.
A better separating effect is achieved by the process disclosed in published Application JP 53 046 944. In this disclosed process, the pure trans,trans stereoisomers of the H.sub.12 -MDI are isolated by treating the starting isomer mixture in organic solvents, for example o-chlorobenzene, with gaseous hydrogen chloride. The carbamic acid chlorides which are formed are less soluble than the free isocyanates and precipitate out from the reaction mixture. This process is disadvantageous because of the technical resources required for continuous separation from the reaction mixture of the carbamic acid chloride solids.
The separation of isomer mixtures of 2,4-TDI and 2,6-TDI by introduction of gaseous hydrogen halide into a benzene solution is described by A. S. Shevlyakov et al. in Sin. Fiz.-Khim. Polim., 5, 66 (1968); CA 70, 3390 h.
The known processes either yield the purified polyisocyanate isomers as solids which can not be continuously worked up without the expenditure of large technical resources, or recover the purified polyisocyanate isomers by distillation, which due to the thermal stress associated therewith may result in contamination with by-products.